Deep penetration laser welding is associated with violent plasma generation characterized by high charge densities. Plasma resides both outside and inside the keyhole, known as plasma plume and keyhole plasma, respectively. Plasma plume outside the keyhole has been studied extensively due to its convenient observation; however, very little work has concentrated on the analysis of the keyhole plasma. In this article, a specially designed setup was used to take firsthand measurements of the light emission of the keyhole plasma in deep penetration laser welding aluminum films clamped in between two pieces of GG17 glass that we called it a “sandwich” sample, triumphantly eliminating the impact of the plasma plume covering the keyhole on the observation of keyhole plasma. Results of spectroscopic measurements of both plasma plume and keyhole plasma under welding conditions were obtained with orthogonal experimental design. It was shown that keyhole plasma had considerable effects on the energy transfer efficiency of the incident laser beam to the material, exhibiting various melting width and depth; deeper welding depth as well as lower temperature of the keyhole plasma was obtained when decreasing the densities of the keyhole plasma by reducing the thickness of aluminum films.
A technique of laser welding glass GG17 (Pyrex) was developed to get clear photographs of the keyhole for quantitative study. The geometrical characteristics of the keyholes are experimentally examined. Based on the known keyhole geometry, the Fresnel absorption and the conduction and convection heat flux loss of the keyhole wall were calculated and compared. A concept of self-regulation of the front keyhole wall for getting energy balance is proposed. The major force that balances the surface tension is also discussed